1. Field of the Invention
The present invention relates to a method for detecting an abnormal portion (defect, non-adhering portion, and seized portion) located at a contact region between each part of a pin and each part of a box forming a threaded joint to be used as a joint of pipes or tubes such as Oil Country Tubular Goods (OCTG). Hereinafter, “pipes or tubes” are referred to as “pipes” when deemed appropriate.
2. Description of the Related Art
Conventionally, as a joint for OCTG, a threaded joint has been widely used. FIG. 1 is an axial directional cross sectional view that schematically illustrates a general structure of a threaded joint. As shown in FIG. 1, a threaded joint 100 is provided with a pin 1 having an external thread part 11, a metal seal part 12, a shoulder part 13 on an outer peripheral surface, and a box 2 having an internal thread part 21, a metal seal part 22, and a shoulder part 23 corresponding to each part of the pin 1 on an inner peripheral surface and being fastened with the pin 1.
The external thread part 11 and the internal thread part 21 (hereinafter, these parts are generally named as “thread parts 11, 21”) are screwed with each other so as to effect a function for fastening the pin 1 and the box 2. The external diameter of the metal seal part 12 is slightly larger than the internal diameter of the metal seal part 22 (this difference is referred to as “an interference margin”), and when the pin 1 is fastened with the box 2, due to the interference margin, a surface pressure is generated on a contact region between the both metal seal parts 12, 22 and due to this contact surface pressure, a function to sufficiently hold an air leakage efficiency of the threaded joint 100 is effected. The shoulder parts 13, 23 effect a function to prevent a high contact surface pressure such that an excess plastic transformation is generated from being generated on the metal seal parts 12, 22 and secure sufficient screwing amount so as to ascertain fastening of the threaded joint 100. Further, not only on the metal seal parts 12, 22 but also on the thread parts 11, 21, the threaded joint 100 may have the same interference margin as the metal seal parts 12, 22 in order to secure screwing of the thread parts 11, 21 so that they are not easily loosened. In this case, the shoulder parts 13, 23 also effect a function to limit the interference margins of the thread parts 11, 21 into a safe area so as to prevent an excess stress on the box 2.
As a method for evaluating a fastening state of a threaded joint having the above-described structure, conventionally, a method for monitoring change of a torque to be generated when fastening a threaded joint has been widely used (for example, refer to Japanese Patent Application Laid-Open No. 10-267175). FIG. 2 is an explanatory view for explaining a conventional method for evaluating a fastening state of a threaded joint. As shown in FIG. 2, as fastening of the threaded joint has been progressed in series, due to a frictional resistance due to interference of the thread parts 11, 21 and interference of the metal seal parts 12, 22, a torque is generated. Then, due to abutting of the shoulder parts 13, 23, the torque rapidly rises. Conventionally, good and bad of the fastening state of the threaded joint is determined by monitoring this change of the torque by an operator. In other words, in the case that the torque rises more than a predetermined threshold value, judging that the shoulder parts 13, 23 abut against with each other, it is determined that the fastening of the threaded joint 100 has been sufficiently completed.
However, according to the conventional evaluating method shown in FIG. 2, the fact that the thread parts 11, 21 interfere with each other, the metal seal parts 12, 22 interfere with each other, and the shoulder parts 13, 23 abut against with each other in face is not evaluated by measuring any physical amount independently and respectively. This is absolutely a method based on a past empirical rule such that a torque is generated because respective parts adhere tightly (interfere or abut) with each other. It is true that a torque is generated when the respective parts adhere tightly (interfere or abut) with each other. However, a large torque may be generated also due to another cause such as seizing of the thread parts 11, 12. Therefore, only by monitoring change of a torque, it is difficult to evaluate the fastening state with a high degree of accuracy (evaluate whether the respective parts 11, 12, and 13 of the pin 1 and the respective parts 21, 22, and 23 of the box 2 are in adhering states or non-adhering states).
Further, even when there is a defect (flaw) that may damage an air leakage efficiency of the threaded joint 100 at the contact region between the respective parts 11, 12, and 13 of the pin 1 and the respective parts 21, 22, and 23 of the box 2 forming the threaded joint 100, it is difficult for change of a torque caused by this defect (flaw) to be generated. Therefore, according to a conventional evaluating method shown in FIG. 2, the defect cannot be detected.
As a method for detecting a defect or an non-adhering portion of a plurality of members, there is publicly known an ultrasonic testing method using reflection of an ultrasonic wave from the defect or the non-adhering portion. Also upon detection of the defect or the non-adhering portion located at a contact region between the pin 1 and the box 2, it may be considered to use this ultrasonic testing method.
Conventionally, for example, as a measurement method using reflection of a ultrasonic wave with respective to a threaded joint of pipes, a method for measuring a contact surface pressure at a contact region between the pin and the box by measuring a reflectance of the ultrasonic wave at the contact region is disclosed in the specification of U.S. Pat. No. 4,870,866.
However, the art described in the specification of U.S. Pat. No. 4,870,866 merely discloses a method for measuring a contact surface pressure at a contact region between a pin and a box, and neither disclosure nor suggestion is made with respect to a method for detecting the defect or the non-adhering portion located at the contact region.
In addition, as an inspection method of a bonded surface of clad steel, JIS G 0601-1989, “testing method of clad steel” using the principle of reflection and passage of an ultrasonic wave on the bonded surface has been put into practical use. More practically, in the above-mentioned JIS standard, an intensity of the first; bottom echo is adjusted to 80%. The first bottom echo is obtained by the first time reflection of the ultrasonic wave passing through a normal portion (namely, bonded portion) in a bonded surface of a plate material from a bottom surface of the plate material. When a portion has the intensity of the first bottom echo not more than 5% and an echo from this portion is mainly received, such a portion is defined as a non-bonded portion.
However, since the above-mentioned JIS standard is a method for inspecting a bonded surface based on change in intensity of the bottom echo of the plate material as described above, this method is difficult to be applied for a threaded joint that cannot obtain the bottom echo. The reason that a threaded joint cannot obtain the bottom echo is because the contact region between the pin and the box is not parallel to the inner peripheral surface of the pin. In addition, since the above-mentioned JIS standard is just a method for detecting a non-bonded portion without assumption that there is a defect (flaw) on the bonded surface, it is difficult to identify whether a reflectance or the like is changed due to existence of a defect on the bonded surface or due to existence of a non-bonded portion (namely, non-adhering portion) on the bonded surface.
It is important for quality control of the threaded joint to identify whether the abnormal portion located on the contact region between each part of the pin and each part of the box forming the threaded joint is a non-adhering portion or a defect. A non-adhering portion may be generated due to shortage of fastening or a dimension error of the box and the pin, or the like. On the other hand, a defect may be generated due to a flaw found on the box or the pin before contact, biting of an impure substance into the contact region, progress of seizing with shortage of a lubricant lying between the box and the pin, or the like. By identifying a non-adhering portion and a defect, it is possible to carry out appropriate quality control in accordance with a kind of the abnormal portion. When the abnormal portion is identified as being a non-adhering portion, it is possible to reconsider a fastening condition or a dimension tolerance of the threaded joint, while, when the abnormal portion is identified as being a defect, it is possible to discard and exchange the defected threaded joint.